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The stethiscope works because it isolates all the ambient noise except for the exact spot you poke the probe. You could probably wear it over earplugs if the noise is too loud

Do you have a smart phone? I'd love to see/hear a video clip (host/post free on YouTube or Vimeo) of this noise!

One warning on Harbor Freight, it is sort of like a toy store for big boys. My last trip I set out to get just a lever grease gun for the side/center stand and with the coupons (sign up online for them, they are awesome) I walked out with a $5 set of punches for doing brake pads, a $4 snap ring pliers with exchangeable sized heads, a $10 torque wrench (that is apparently surprisingly good), a $3 set of picks, $5 set of ball end metric Allen wrenches, $4 organizer for various sizes of crush washers, $4 magnetic pickup wand, $3 magnetic cup bolt holder, etc. etc. etc. If you walk in with a shopping cart, you're doomed

I already thought of that, Lee but the trouble is the noise doesn't show up until I've really warmed up the bike (as in lots of stop and go traffic or on a hot day)---so I don't feel comfortable with an open tank while riding around town. I just filled her to the brim with Chevron regular (her favorite brew), too.

I do have a spare which I will fetch as soon as I get going this morning. I'm heading up to Harbor Freight which is just the right distance for getting the screech to happen as I pull back into the driveway...

Harbor Freight stethoscope, etc.

The stethiscope works because it isolates all the ambient noise except for the exact spot you poke the probe. You could probably wear it over earplugs if the noise is too loud

Do you have a smart phone? I'd love to see/hear a video clip (host/post free on YouTube or Vimeo) of this noise!

One warning on Harbor Freight, it is sort of like a toy store for big boys. My last trip I set out to get just a lever grease gun for the side/center stand and with the coupons (sign up online for them, they are awesome) I walked out with a $5 set of punches for doing brake pads, a $4 snap ring pliers with exchangeable sized heads, a $10 torque wrench (that is apparently surprisingly good), a $3 set of picks, $5 set of ball end metric Allen wrenches, $4 organizer for various sizes of crush washers, $4 magnetic pickup wand, $3 magnetic cup bolt holder, etc. etc. etc. If you walk in with a shopping cart, you're doomed

Good morning Ted; I'm heading to HF in a little bit to get the stethoscope and will endeavor to use it without losing my hearing---the over earplugs is a good idea, at least for starters.

I suppose I could stick it in the gas tank and listen to the fuel pump even though the tank is full (I'll make sure the 'scope is clean). Then I can try the new FPR just to be double sure. I'll check the water/oil pump next and then around the alternator (I wonder if monkey nuts make such a high-pitched screeching sound? Kind of doubt it but I've never heard the sound that they make when they start to go).

No, I'm sorry but I don't have a smart phone but I might be able to make a cassette recording that could possibly be uploaded---we'll see what I can learn with the stethoscope and go from there.

Your comments about Harbor Freight are classic. Should be made into a sticky!

After the first time I went there this summer I told my wife, "It's like Trader Joe's for mechanical stuff!"

I, too, came home with a bunch of stuff that I couldn't pass up.

Worth mentioning is their 5lb. capacity vibratory tumbler that was on sale for $55 (vs. $100 reg.). I've used it to de-rust all the non-stainless steel bolts on my bikes and assorted rusty bits and tools that had suffered "rain damage" from living in Hawaii too long. Makes the steel and galvanized bits look like new but doesn't wear down the threads. Only takes an hour or two in a couple pounds of hard plastic tumbling media that they also sell in huge jars.

Well, I'll post a report here as soon as I have further developments. Thanks again for your help.

Video possibility

PS: Just remebered that we have an older point-and-shoot digital camera that can take video; I'll add that to my to do list and see if I can post it on YouTube (unless I find something obvious with the stethoscope beforehand).

I think you will be able to identify the sound on the outside of the tank, you should be able to hear the resonation if it is the pump inside. Literally just keep poking the end around until you isolate the source of the sound. It is sort of like trying to find an air leak, just go slow and methodically.

I'm back from Harbor Freight and it was a beautiful ride both ways. I took the windscreen off when I was taking the tupperware apart to do some rewiring for my auxiliary lights and decided not to put it back on until the weather turns really cold.

If it weren't for this noisy problem I'm encountering I would say that my bike is absolutely running the best it ever has. I had the valves adjusted a couple of weeks ago at the dealer's and I also had them put in new plugs for good measure.

It's so smooth that I wanted to keep on riding around Albuquerque, zooming down the wide boulevards in the cool fall afternoon but I needed to get back and attend to another minor problem first (fixing one of my bag's latches that broke off when I pulled too hard).

I got out of HF for under $15 but it was not easy there are so many temptations that await you!

I got the $4 stethoscope, a $6 pair of large 3X power magnifying glasses that clip on to your regular glasses (for close up electronic repair that I do sometimes) and a $3 box of 3/16th" rivets that I plan to use for the latch.

Im going outside to start on the latch and then I'll take my wife to work on the bike a little later. That should warm it up enough so I can induce the noise long enough to start probing the tank and engine casing for the source of such a racket.

Good to know I won't need to actually probe inside the tank, Ted, too---I'll try the lower right outside of the tank first screech that comes on parked at idle.

Rivets and SCREECHING DIAGNOSIS AT LONG LAST!

The smaller rivets inside the cases are 1/8th as that's what I used several months ago to replace a couple; they matched perfectly. But the large latches sure looked bigger so that's why I went with 3/16ths. My latch is now back on the bag securely and it's working fine again. Yes, I did have to drill out the old rivets first which was a bit tricky but I think I'm good to go now.

Maybe the OEM size is 4.0mm which may be right in between 1/8th and 3/16ths?

On the screeching front I reached a diagnosis about 5 minutes ago. After finishing the latch job I put my gloves and helmet on and set out for an around town ride to get the engine well-hot, mon (as they say in Jamaica).

Not so sure if I should jump for joy as it's still going to be a major task for me to fix but it's nothing to do with the fuel system after all!

After riding around for twenty minutes the noise came back as I came to a stoplight near my home.

With the bike running, I pulled over on my street, took off my helmet and quickly got out the stethoscope. (Works great, Ted! Thanks for the suggestion)

Fuel pump? No.

Alternator? No.

Engine block? No.

Oil/water pump? No, thank dog!

New FPR? No, so the old one is probably still fine. Now I have two good spares besides the new one I just installed!

The culprit? The swan song of the radiator fan bearings is the noise I've been hearing all this time!

I'm surprised I didn't suspect the fan a lot more than I did because it would always happen when conditions were hot enough for the fan to engage.
While I could have sworn that there were times the noise wouldn't be there when the fan was on and vice-versa; I didn't take into account that it would be hard to hear the regular fan noise at speed. Only the chirping would get through and was especially prominent at stoplights or slow traffic.

So now I will look into replacing the fan which seems like both an expensive and daunting task. I don't suppose there is a way to save it instead of replacing it, is there?

I'll check MaxBMW first to get the major price jolt, then Beemer Boneyard and Drake's website up in Seattle for used ones that might be available at lower cost.

That's the news for today. With the cooler weather on its way I suppose I can still ride the bike as is for a while longer, at least. I should have known that the big clue I was overlooking was the fact that the noise only popped up when it was hot outside or on the bike. And the fan was on when it was happening a lot.

Thanks for all the support and interest along the way, guys. I'm sorry it took so long to figure it out.

I'll be standing by for any further advice on how I should go about repairing or replacing the fan.

Jeff - your best bet - visit a Harbor Freight and purchase a mechanic's stethoscope. That should let you localize it.

"Under the tank" is a wide expanse of territory to guess where the noise is coming from. It could be anything from a bearing going bad somewhere, to the a plugged fuel filter (the fuel pump has a blow-off valve that chirps when it's pushing against too high a resistance.)

Right you are, Don. In my dilemna and advancing age I got sidetracked by the blocked fuel return line and actually forgot about the stethoscope until Ted piped in about it later on. I am getting a bit more senile at age 61 than I would have expected (absent-minded more than ever lately; I think my 88 year-old dad is sharper than I am!

Oh, well, at least I have an almost completely new fuel system going and the bike is running great otherwise (and I didn't damage anything important by operating the bike while screeching; it was only a fan motor bearing).

I just noticed that Beemerboneyard sells just the fan motor (Bosch!) for $79 so I immediately ordered one minutes ago. Now I'll look into what I need to remove in order to install it asap...

Thanks again for your help and I'm sorry I spaced out acting on your excellent advice.

Well what do you know about this...!

Another slightly incorrect diagnosis cropped up this afternoon as I removed the radiator assembly and loosened the two bolts holding the fan in place.

When I swung the fan up and out away from the back of the radiator I discovered that a small section of the radiator elements were bent and mashed down maybe 1/8th of an inch---looks like the fan motor shaft was rubbing itself into the radiator! Maybe the motor bearings were not bad after all (will hook up to 12v and test soon)

Fortunately, when I drained the radiator earlier the Mercedes (or maybe it was Swiss) blue anti-freeze I had put in about three years ago still looked very clean and translucent. There was also plenty of it---so no apparent leaks.

Tomorrow I'll take and post a picture of the radiator damage and hope to get some opinions on how to best avoid a repeat scenario after I install the new fan motor. I'll also try to gingerly pry the radiator fins apart and straighten them out as best I can. Otherwise the radiator seems to be in great shape for an 18 year old bike.

Even if the old motor is OK I'll still gladly install the new one. Here in the desert I can probably expect a slightly longer fan life and my K will be all the better in the long run.